A conventional brush device with a resilient means to impel the brush against the commutator (see Patent Document 1, for example) is shown in FIG. 16. This brush device 24 has a brush 19 formed in a substantially arc shape, which is supported by contact between part of its outer surface and the inner wall of a holder 22.
The outer face of the brush 19 at the end facing the commutator 2 is made to contact the inner wall of the holder 22 by the force of friction with the commutator 2, and the inner edge of the brush 19 at the end facing the torsion coil spring 23 is made to contact the inner wall of the holder 22 by impulse of the torsion coil spring 23. In this way, the brush 19 is supported within the gap g between the inner walls of the holder 22, and its ability to follow the commutator 2 is enhanced. Further, because the brush 19 is formed in an arc shape, the brush friction is maintained by rubbing and the overall diameter of the brush device is minimized.
However, there are still problems in the brush device 24 shown in FIG. 16, in regard to assuring reliable electrical conductivity between the commutator and the brush. In FIG. 16, the brush 19 is held just by providing the gap g between the inner walls of the holder 22, and the commutator 2 comprises, as an actual product, a number of commutator blades 2a. Accordingly, when the brush 19 straddles the space between the commutator blades 2a, the support of the brush 19 is destabilized and vibration is liable to occur.
Further, depending on the application of the motor with a bush, the commutator will sometimes be driven in opposite directions, clockwise and counter-clockwise. With the brush device in FIG. 16, however, if the direction is reversed and the commutator is driven clockwise, the brush 19 is liable to drift because of its two-point support in the gap g, and the instability of support of the brush 19 is liable to cause vibration.
The brush device 14 shown in FIGS. 17 and 18 was put into production in order to resolve the problems described above. In the case of this brush device 14, the commutator comprises commutator blades 2a mounted on the outer periphery of the motor shaft 1, separated by equal gaps, and the carbon brush 10 is fitted to and held by the support post 7 of the brush holder 4. This brush 10, as shown in FIG. 17, has a rubbing surface with electrical conductivity when in contact with the commutator blade 2a. The brush 10 has the arched brush arm 10a and the brush base 10b with the hole 8 through which the support post 7 passes for mounting the brush 10 on the brush holder 4. By forming the brush arm 10a and the brush base 10b as a single unit, the brush 10 is formed in substantially a V shape. The coil spring 5 or other means of impulse is mounted on the support post 7 and concentric with it, so that the rubbing face of the brush 10 is impelled against the commutator blade 2a and held in contact with the commutator 2.
Because the brush device 14 has a means of impulse such as the coil spring 5 (only one of the two brushes 10 is shown in FIG. 18), changes to the force of contact from friction between the brush 10 and the commutator 2 are prevented, and so there is superior stability of electrical conductivity between the commutator 2 and the brush 10. Moreover, there is the advantage that the brush arm 10, which corresponds to brush 19 in FIG. 16, is supported by the relatively large brush base 10b, and because this brush base 10b is fitted to the support post 7, the stability of brush support is greater than in the brush device of FIG. 16, so that the occurrence of vibration is inhibited when the motor is driven.
Nevertheless, the structure of the brush device 14 is such that the brush 10 is mounted over the coil spring 5, which is the means of impulse, and so the thickness of the brush device 14 increases that much more in the direction of the motor shaft 1. Consequently, there is the problem that miniaturization of the brushed motor in the direction of the motor shaft is impeded.
The present applicants earlier applied, in Japanese Patent Application No. 2001-367265, for the brush device 11 as shown in FIGS. 19 and 20. This application provides the guide 15 as part of the brush base 3b, as well as the recess 6 in the brush holder 4. Within this recess 6, one end of the coiled portion of the coil spring 5 is stopped by the stop face 3c of the brush base 3b, and the other end is stopped by the step 4a of the brush holder 4. The longitudinal direction of the coil spring 5 is aligned with the face of the brush holder 4 (the direction shown by arrow C in FIG. 20), and the winding follows the wall of at least one of the guide 15 and the recess 6; the coil spring 5 and the brush 3 are in substantially the same plane. This is the essence of the brush device 11.
By constituting the brush device 11 as described above, the longitudinal direction of the coil spring 5 is located in substantially the same plane as the face direction of the brush holder 4, and so the brush device can be made thinner than earlier brush devices. Further, by applying this brush device 11 in a motor with a brush, it is possible to reduce the length of the motor shaft 1 without impairing the characteristics of the brushed motor.
Patent Document 1: Japanese Patent Application Laid-Open No. H07-213023 (pages 3 and 4, FIG. 2)